(a) Field of the Invention
The present invention relates to an audio player that uses a sigma-delta pulse-width modulation (Δ-Σ PWM) architecture and, more particularly, to a Δ-Σ PWM audio player having a considerably reduced signal error.
(b) Description of the Related Art
FIG. 1 shows a schematic view illustrating a conventional audio player 10 that uses a sigma-delta pulse-width modulation (Δ-Σ PWM) architecture. A typical Δ-Σ PWM audio player includes a Δ-Σ PWM unit 11, a drive unit 12, and a speaker unit 13. The Δ-Σ PWM unit 11 receives input signal and generates a PWM signal y, which is a data sequence consisting of values 1, 0, and −1. The drive unit 12 receives the PWM signal y and generates analog output signals AUDP and AUDN to drive the speaker unit 13.
FIG. 2 illustrates a circuit architecture for the Δ-Σ PWM unit 11. The Δ-Σ PWM unit 11 is used to generate a PWM signal y according to an input signal, where the PWM signal is a feedback signal that serves for adjusting the sound quality of the audio player 10. The Δ-Σ PWM unit 11 includes four adders 111a, 111b, 111c and 111d, two delay units 112a and 112b, two amplifiers 113a and 113b, a limiter 114, and a quantizer 115. The quantizer 115 receives an output signal Z0 of the adder 111c. If the signal Z0 is larger than a certain threshold C0, the PWM signal y is set as 1; if the signal Z0 is smaller than a negative threshold −C0, the PWM signal y is set as −1; Otherwise, the PWM signal y is set as 0.
Referring to FIG. 2, the amplifiers 113a and 113b of the Δ-Σ PWM unit 11 receives the PWM signal y from the quantizer 115 as a feedback signal. Hence, the value of the feedback signal for the Δ-Σ PWM unit 11 is the ideal output value, namely 1, 0 or −1. However, in real condition, the value of the feedback signal is not the ideal 1, 0 or −1, because an actual feedback signal must contain some distortion or error contributed by the drive unit 12 under different loadings. Hence, compared to theoretical values, the signal noise ratio (SNR) and total harmonic distortion (THD) in real condition are not good because the feedback signal must contain some distortion or error that varies with actual loading.
FIG. 3 illustrates a Δ-Σ PWM audio player that uses an analog to digital converter (ADC) to generate a feedback signal, and, by means of this architecture, the error that varies with actual loading for the feedback signal is taken into account. Referring to FIG. 3, the Δ-Σ PWM audio player 30 includes a Δ-Σ PWM unit 11′, a drive unit 12, a speaker unit 13, and further an ADC 34. The ADC 34 is used to convert the output signals AUDP and AUDN of the drive unit 12 into digital signals that serve as the feedback signal of the Δ-Σ PWM unit 11′. Thereby, the feedback signal for the Δ-Σ PWM unit 11′ is the actual output signals of the drive unit 12 rather than the ideal PWM signal y, so that the Δ-Σ PWM audio player 30 may eliminate the error of the output signals of the drive unit 12.
FIG. 4 illustrates a circuit architecture of the Δ-Σ PWM unit 11′. The components of the Δ-Σ PWM unit 11′ are the same as those of the Δ-Σ PWM unit 11, except the amplifiers 113a and 113b of the Δ-Σ PWM unit 11′ receive actual output signals FS generated from the ADC 34 as the feedback signal.
However, the high resolution of a typical Δ-Σ PWM unit causes the ADC to have the same requirement of high resolution (16-bit or above), and the high resolution ADC is expensive to result in a considerable cost. On the other hand, a typical Δ-Σ PWM unit tends to suffer considerable noises and has a high processing speed, and thus the ADC must be designed to have adequate capability of anti-noise and have a high procession speed. This may result in a high degree of design difficulty for the overall circuit.
As described above, due to the considerable cost and design difficulty, it is less practical to eliminate the error of the output signals by means of the ADC.